Remove inconsistent use of EOPNOTSUPP

Commit 3ee56c292b changed an ENOTSUP return value
in one location to ENOTSUPP to fix user programs seeing an invalid ioctl()
error code.  However, use of ENOTSUP is widespread in the zfs module.  Instead
of changing all of those uses, we fixed the ENOTSUP definition in the SPL to be
consistent with user space.  The changed return value in the above commit is
therefore no longer needed, so this commit reverses it to maintain consistency.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/zfs_ioctl.c

@@ -3291,7 +3291,7 @@ out:
 		dsl_dataset_rele(ds, FTAG);
 	return (error);
 #else
-	return (EOPNOTSUPP);
+	return (ENOTSUP);
 #endif /* HAVE_ZPL */
 }